This invention relates to a photoelectric conversion device using Charge Modulation Devices (hereinafter abbreviated as CMDs) or photoelectric transducers which have a like amplifying function.
While a solid state image sensor using CMDs as pixels is known to the public as disclosed in IEDM Tech. Dig. pp. 353-356, 1986, the basic operation of reading an image pick-up signal will be first explained. FIG. 1 is an equivalent circuit diagram showing a state when the signal is read out of one CMD. A gate 4 of a CMD 1 providing each pixel is connected to an output terminal of a gate control circuit 5, a drain 2 of the CMD 1 is connected to a drain bias (not shown), and a source 3 of the CMD 1 is connected to a current/voltage conversion circuit with a virtual ground input which comprises an operational amplifier 6 and a feedback resistor 7.
FIG. 2 is a chart of signal waveforms for explaining basic readout operation of the CMD solid state image sensor shown in FIG. 1. The gate control circuit 5 in FIG. 1 outputs a gate control pulse .PHI..sub.G comprising three values; i.e., integrating potential V.sub.INT, readout potential V.sub.RD and reset potential V.sub.RST, as shown in FIG. 2. If the CMD 1 is exposed by a beam of light 10 under a condition that the integrating potential V.sub.INT is applied to the gate, the source current corresponding to an exposure amount so far exposed flows through the CMD 1 when the gate potential is set to the readout potential V.sub.RD.
Such a behavior is depicted in a graph of FIG. 3 showing operating characteristics of the CMD. A characteristic curve 9-1 corresponds to a source current characteristic when the exposure amount is zero, i.e., i n a dark state, and characteristic curves 9-2, 9-3 correspond to respective source current characteristics as developed when the exposure amount is gradually increased. Therefore, assuming that the feedback resistor 7 has a resistance value R.sub.f, output voltage V.sub.OUT at an output terminal 8 of the current/voltage conversion circuit is given below for each of the characteristic curves 9-1, 9-2 and 9-3: EQU V.sub.1 '=-I.sub.1 .multidot.R.sub.f ( 1) EQU V.sub.2 '=-I.sub.2 .multidot.R.sub.f ( 2) EQU V.sub.3 '=-I.sub.3 .multidot.R.sub.f ( 3)
However, the technique of reading a source current which flows when the read potential V.sub.RD is applied to the gate terminal, as stated above, has suffered from the disadvantages described below. Because a potential change in the gate field surface of the CMD caused depending on the exposure amount modulated and outputted in accordance with the CMD's transistor characteristics, photoelectric conversion characteristics deviate from a linear line, and the sensor output may fluctuate in spite of the exposure amount held constant if temperatures are changed. Another disadvantage is that because the CMD's characteristics for modulating potential change in the gate field surface, caused depending on the exposure amount, into a source current are different from pixel to pixel, variations in the characteristics of each CMD lead to the fixed pattern noise in a video output signal and the performance of the image sensor is deteriorated.